It is known to use such sheet metal strips and such collecting electrodes in dry-process dust-collecting electostatic precipitators.
In many cases, such electrodes define alternating or symmetrically disposed dust-collecting pockets (see German Patent Publication DE-AS 11 07 203, German Utility Model DE-GM No. 18 48 928) and the electrodes are often so designed on their vertical longitudinal edges that consecutive sheet metal strips can be hooked into each other (see published German Application No. DE-OS No. 17 71 478, German Utility Model DE-GM No. 18 51 222, German Utility Model DE-GM No. 18 69 720, Austrian Patent No. 277,400 and Swiss Patent No. 486,920).
Some of the known collecting electrodes are positively connected directly to a rapping mechanism (German Utility Model DE-GM No. 18 51 222) so that the linkage of this mechanism constitutes a transverse connector.
Other pertinent details of prior art systems have been disclosed in the following printed publications: German Patent Specification No. 316,703, published German Application No. 14 07 529, published German Application No. 33 20 360, European Patent Specification No. 14,273, French Patent Specification No. 21 26 068 and U.S. Pat. No. 3,282,029.
The known sheet metal strip collecting electrodes have various disadvantages. For instance, the collecting electrodes disclosed in German Patent Publication DE-AS No. 11 07 203 comprise pairs of profiled sheet metal strips which have been joined by welding. The profiled sheet metal strips which have been cold-worked may be distorted during the welding operations and this distortion may result in a change of dimensions particularly if the strips are very long.
Such changes of dimensions cannot be tolerated where the spacing from the corona electrodes is to be precisely predetermined.
Another disadvantage of many known collecting electrodes is that dust-collecting pockets can be provided only on one side of each corona electrode so that the corona electrodes need be provided with corona tips only on one side.
In that case the dust will be less efficiently collected on those collecting electrode surface areas which do not face a corona tip and, when the collecting electrodes are cleaned by being rapped, the less efficient surface portions will cause a raising of a substantial portion of the dust that has already been collected. A large part of the dust thus released will be entrained with the gas stream so that the efficiency of the dust collector will be reduced.
In addition, the electrostatic precipitator must be assembled with great care because the corona tips must exactly be aligned with the dust-collecting pockets since an inadequate alignment of the corona tips will necessarily result in a reduction of the dust-collecting rate to a fraction of the normal rate.
Some of the disadvantages described hereinbefore have been avoided with the collecting electrode disclosed in German Utility Model DE-GM No. 18 48 928. But even with these electrodes two successive electrode strips are simply hooked into each other in a less-than-positive connection. In addition the sectional shape of the profiled strips does not permit them to be positively connected directly to the rapping linkage. Finally, the dust-collecting pockets have depths which are rather small in relation to their width so that the rapping may release a large part of the previously collected dust and the raised dust can be entrained by the gas stream. That effect is particularly significant at the last electrode strips of a dust collector because dust released here cannot be subsequently collected so that the dust content of the pure gas will be increased.
The collecting electrodes disclosed in published German Application DE-OS No. 17 71 478 also have most of the disadvantages which have been mentioned. In these electrodes it is highly undesirable that each sheet metal strip define only a single dust-collecting pocket and that adjacent strips can be hooked one into the other at their vertical longitudinal edges only if the profiled strip is made to very small tolerances in that region. This is hardly practicable in the manufacture of conventional sheet metal strips which can have lengths of up to 15 meters. As a result, it is difficult to assemble the strips and these difficulties can be avoided only if high costs are incurred; nevertheless, the results are often unsatisfactory.
The substantial deformation of the material as it is given by its profiled shape is a major disadvantage of the collecting electrodes disclosed in German Utility Model DE-GM No. 18 51 222, German Utility Model DE-GM No. 18 69 720 and Austrian Patent No. 277,400.
Such high-degree deformation will inevitably result in work hardening, which when the electrodes have been in operation for a prolonged time will give rise to the formation of cracks under the dynamic stresses due to the rapping blows applied to clean the electrodes.
While the electrode stips disclosed in the latter publications can be positively connected at their lower end to the rapping linkage, as is shown in German Utility Model No. 18 51 222, the hooked joints between the vertical longitudinal edges of adjacent electrodes are not adequate for a prolonged operation involving temperature fluctuations, particularly if the electrodes have the abovementioned large lengths that are usually prevalent.
The approaches used in the remaining above-mentioned publications also have failed to provide fully satisfactory collecting electrodes and collecting conditions.
Extensive investigations have shown that a considerable number of controlling parameters conflicting with each other must be properly selected for a fully effective collecting electrode.
For instance, the depth of the dust-collecting pockets must be so large that said pockets can form low-flow regions which are substantially free from the influence of the gas stream in operation so that dust will be effectively collected, but yet the raising of dust by the rapping blows applied to clean the collecting electrodes will be minimized.
However, with given overal flow cross sections the velocity of the gas flow increases as the flow cross section is decreased by deeper dust-collecting pockets. But this will necessarily result in a higher pressure drop and will give rise to the danger that the optimum velocity of flow of the gas cannot be maintained and the dust-collecting rate is reduced.
Moreover, the collecting electrodes should ensure that the flow density distribution will be as uniform as possible. In the most ideal case this is achieved with tubular collecting electrodes and centrally disposed corona electrodes, and it is desirable to approach this effect with plate-like collecting electrodes.
Finally, the sheet metal strips for the collecting electrodes should be suitable for an economical manufacture and should not have strongly profiled portions which are susceptible to stress corrosion. Besides, they should have a satisfactory relation between the effective collecting surface area and the expenditure of material and should be adapted to be assembled, repaired and replaced easily.